The present invention relates to the field of mass storage devices. More particularly, this invention relates to an improved apparatus and method for writing servo information to the disc of a high density disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc drive housing, a disc that is rotated, an actuator assembly that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
To read and write data to the disc drive, the actuator assembly includes one or more arms that support the transducer over the disc surface. The actuator assembly is selectively positioned by a voice coil motor which pivots the actuator assembly about a pivot shaft secured to the drive housing. The disc is coupled to a motorized spindle which is also secured to the housing. During operation, the spindle provides rotational power to the disc. By controlling the voice coil motor, the actuator arms (and thus the transducers) can be positioned over any radial location along the rotating disc surface.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equalize so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on portions of the storage disc referred to as tracks. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write to or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is often divided between several different tracks. While most storage discs utilize a multiplicity of concentric circular tracks, other discs have a continuous spiral forming a single track on one or both sides of the disc.
During manufacture, servo information is encoded on the disc and subsequently used to accurately locate the transducer. The written servo information is used subsequently to locate the actuator assembly/transducer head at the required position on the disc surface and hold it very accurately in position during a read or write operation. The servo information is written or encoded onto the disc with a machine commonly referred to as a servo track writer (hereinafter STW). At the time the servo information is written, the disc drive is typically at the xe2x80x9chead disk assemblyxe2x80x9d (hereinafter HDA) stage. The HDA includes most of the mechanical drive components but does not typically include all the drive electronics. During the track writing process, the STW precisely locates the transducer heads relative to the disc surface and writes the servo information thereon. Accurate location of the transducer heads is necessary to ensure that the track definition remains concentric. If the servo track information is written eccentrically, the position of the transducer head during subsequent operation will require relatively large, constant radial adjustments in order to maintain placement over the track center. When the tracks are sufficiently eccentric, a significant portion of the disk surface must be allotted for track misregistration.
When servo information is encoded or written to the discs of the disc drive several items are very important. First of all, the radial position of the servo marks must be carefully controlled so that the disc drive can always get an indication of the radial position of the transducer with respect to the disc. As a result, motor speed is carefully controlled in current servo writers. If the motor speed is not maintained at a selected RPM (revolutions per minute), repeatable run out error can result in the servo written disc drive. Secondly, the servo writer should not write in non-repeatable run out. This can happen if portions of HDA are vibrating during the servo writing operation. For example, the actuator may resonate at one rotational frequency. At other rotational frequencies, there can be spindle motor gyro effect or disc fluttering.
A constant desire or industry goal is to increase the storage capacity of disc drives. One way to increase capacity is to increase the density of tracks on the disc. Currently, the number of tracks per inch (TPI) is growing at a rate of 60% per year. It is contemplated that this trend will continue and further more that the percentage growth may even increase.
During manufacture, the time required to servo write a disc drive is lengthy. With the increased number of tracks on a disc and the constant trend to increase the number of tracks per inch, the length of time for servo writing discs will also increase. Another pressure is to cut down production time for disc drives. One way of doing this is by servo writing the disc while spinning the disc at a higher RPM than the designed spindle speed of the disc drive.
Presently, the selection of a rotational speed for the disc drive is severely constrained or limited by several factors. Present designs of servo writers do not allow for easy, flexible selection of the servo writer spindle motor speed. Spindle motor speed selection for a servo writer is generally constrained by many factors including the spindle motor gyro effect, disc flutter, the servo clock and the servo pattern. There are other factors that also may influence speed selection. Normally, the spindle motor is optimized at the disc drives operating speed so that the above factors are minimized at the operating spindle speed. At many of the spindle motor speeds other than the operating spindle motor speed, a bearing defect may very well induce the spindle motor gyro effect. The spindle motor gyro effect will generally cause a non-repeatable run out. This non-repeatable run out will be xe2x80x9cfrozenxe2x80x9d during servo writing process, subsequently seen by the drive as written in repeatable run out. At other speeds, other factors may result in either non-repeatable run out (NRRO) or repeatable run out (RRO) in a written servo pattern.
Typically, there are other xe2x80x9cgoodxe2x80x9d spindle speeds where the factors that cause either non-repeatable run out (NRRO) or repeatable run out (RRO) during servo writing process will be minimized. However, many times the xe2x80x9cgoodxe2x80x9d spindle speeds do not match the frequency at which the servo writer must write to produce the desired servo pattern.
To servo write at the xe2x80x9cgoodxe2x80x9d speed, the servo writer clock may have to run at some odd frequency that is impossible for most of the servo writer. Usually the servo writer clock is driven by a crystal and PLL (Phase lock loop) which typically has limited selections.
To servo write at higher spindle speed, normally the servo clock frequency is increased to the next higher clock from the normal operating clock. This will increase the spindle motor speed proportionally without altering the original servo pattern format. For example, the operating speed for the drive is 5400 rpm and the servo clock is 50 MHZ. If the next higher servo clock available is 60 MHZ, then the spindle motor speed will be (60*5400/50) rpm or 6480 rpm. If the new speed does conflict with the gyro frequency, then check at the next servo clock frequency and the step is repeated until a good speed is found or reaches the maximum servo clock frequency. However, if the calculation shows a match between the servo frequency and a good higher motor speed, the spindle motor may not be able to run at the speed due to the strong spindle motors"" bemf and motor driver saturation.
Accordingly, what is needed is an apparatus and method for use with a servo track writer (STW) that will allow the servo patterns to be written at any speed of the spindle motor so that a spindle speed may be selected that minimizes the factors resulting in either non-repeatable run out (NRRO) or repeatable run out (RRO). What is also needed is a servo writing technique that allows the servo to be written at higher spindle speeds than the operating spindle speed so that the total manufacturing time can be reduced by increasing the throughput of the servo write operation. What is also needed is a method and apparatus which uses the current servo writing equipment.
The present invention is directed to reducing these problems, especially the non-repeatable track writing errors due to imperfect bearings. The invention is a method for writing servo patterns on at least one disc in a disc drive. The method includes the steps of spinning the disc at a first selected rotational speed and writing a clock track on the disc at the first selected rotational speed. The method further includes spinning the disc at a second rotational speed while tracking the written clock track pattern on the disc using the servo clock phase lock loop circuitry as the disc rotates at the second rotational speed. Servo information is written while the disc is rotated at the second rotational speed. The second rotational speed may be selected so that it is faster than the first rotational speed. The second rotational speed is selected to minimize the causes of repeatable and non-repeatable run out in the disc drive. The method may further include the step of selecting the second rotational speed which is faster than the first rotational speed, said second rotational speed selected so as to minimize the causes of repeatable and non-repeatable run out in the disc drive. The second rotational speed is selected so as to minimize a spindle motor gyro effect in the disc drive. The method also includes writing the servo information using the clock signal generated by phase lock loop circuitry tracking the clock track on the disc. The servo patterns are generated in response to the clock signal generated by phase lock loop circuitry tracking the clock track on the disc. The method can also include generating servo patterns in response to the clock signal generated by phase lock loop circuitry tracking the clock track on the disc, and writing the generated servo patterns to the disc at an area other than the area of the disc having the clock track. The method also includes writing the generated servo patterns to each surface of the disc at an area other than the area of the disc having the clock track. The disc will have two surfaces.
Also disclosed is a servo track writer for writing servo information to a head disc assembly which includes at least one disc and a transducing head. The servo track writer includes a clock head placed in transducing relation to the disc. The clock head writes a clock track to the at least one disc and generates a clock signal when the clock head reads the clock track on the at least one disc. The invention also includes a servo pattern generator that writes servo patterns to the at least one disc in response to the clock signal generated by the clock head reading the clock track on the at least one disc. The servo track writer further includes a phase lock loop clock generator which synchronizes to the clock track and produces the clock signal for the servo pattern generator. The servo pattern generator writes with the transducing head. The at least one disc has a first surface and a second surface. The servo pattern generator writes to the first surface and the second surface of the disc. The servo pattern generator may write an embedded servo information pattern to the at least one disc with the transducing head.
The servo track writer further includes a mounting fixture for mounting the head disc assembly. The mounting fixture provides a mechanical reference for the servo track writer as servo information is written to the head disc assembly. A positioner for controlling the position of an actuator of the head disc assembly is also included. The servo track writer also includes a spindle controller for controlling a spindle motor of the head disc assembly. The spindle controller is capable of controlling the spindle at a first operating speed and a second servo writing speed. The second servo writing speed may be selected to be faster than the first operating speed of the spindle. The servo track writer further includes a controller for controlling the positioner, the servo pattern generator, the phase lock loop clock generator, and the spindle controller.
Most generally, the invention is to a servo track writer for writing servo information to a head disc assembly includes a servo pattern generator, and a device for writing servo information to the disc while the disc rotates at a speed other than the operating speed of the disc drive.
Advantageously, the method and apparatus of the present invention allows servo patterns to be written to a disc at any speed of the spindle motor using a servo track writer (STW). In other words, a spindle speed that minimizes the factors resulting in either non-repeatable run out (NRRO) or repeatable run out (RRO) from the servo track writer (STW) is selected. The servo writing technique allows the servo to be written at higher spindle speed than the operating spindle speed so that the total manufacturing time can be reduced by increasing the throughput of the spindle write operation. A further advantage is that the method and apparatus uses the current servo writing equipment. The equipment is used in a different way so that there is little, if any, additional equipment cost resulting from using the invention.